Police arrested 31 as they clashed with protesters in another night of gunfire, teargas and chaos in Ferguson 10 days after the shooting of an unarmed teenager ignited an uproar over race in America.

A massive show of force by riot police and newly arrived national guard units failed to quell agitators who fired gunshots and threw Molotov cocktails on Monday night and early on Tuesday morning.

Missouri national guard troops entered the protest zone and police ordered the media to leave as officers in armoured vehicles fired teargas and stun grenades and engaged with gunmen.

The mayhem dashed hopes that the deployment of the national guard, and greater community efforts to control the small minority of violent protesters, would ease a crisis which flared after a local police officer shot Michael Brown, 18, on 9 August.

At a 2.20am press conference, Captain Ron Johnson of the Missouri state highway patrol said 31 people had been arrested, some who had come from as far afield as New York and California. He said two people were shot and displayed two handguns and a Molotov cocktail he said had been confiscated by police.

Police fire tear gas in the direction of where bottles were thrown from crowds gathered near the QuikTrip. Photograph: David Carson/Polaris/eyevine

Johnson made an impassioned defence of the scale of the police response, which has come under sharp criticism. He said officers had acted with restraint against “criminal acts by a tiny minority of lawbreakers”.

Adding that police had at one point come under gunfire, and that several officers had been injured by rocks and bottles, Johnson urged peaceful protesters from now on to limit their demonstrations to daytime and not give cover to criminals at night.

“We’re going to make this neighbourhood whole, we’re going to make this community whole, and we’re going to do it together,” said Johnson, close to tears. “And I am not going to let criminals who come here from across the country or live in this community define this neighbourhood.”

A demonstrator protects himself from teargas at the protest. Photograph: Scott Olson/Getty Images

Monday’s protest remained peaceful until about 10pm. More than a thousand people, including dozens of Pentecostal ministers, marched without incident under the gaze of police with helmets and shields.

When rowdy youths threw bottles of water at police, volunteers formed a line and herded them back, away from police. They begged the officers to not intervene: “Relax, we’ve got this, give us a chance.”

But more serious trouble flared 500 metres away at the site of a burned convenience store where dozens of youths, some with covered faces, ripped up street signs and taunted police. Unidentified gunmen opened fire, triggering volleys of police stun grenades and teargas.

Provocateurs had again infiltrated a legitimate protest, Ron Conway, a highway patrol captain who commands policing in Ferguson, told CNN. “That element that has been causing havoc got within the peaceful protest.” Two people were shot and were taken to hospital, he said.

The fiercest clashes took place near the site where a police officer, Darren Wilson, shot Brown. Journalists were unable to monitor the scenes after being ordered to a command centre about a mile away. Several journalists were detained, apparently for unlawful assembly.

Police at the demonstration in Ferguson. Photograph: Scott Olson/Getty Images

Conway defended the use of militarised force. “We can’t send officers in squad cars in areas where they’re shooting bullets. We have to send them in armoured cars.”

The Missouri governor, Jay Nixon, struggling to find an effective response to the protests that have rocked Ferguson since the shooting of Brown, has abandoned a controversial and ineffective curfew that failed to prevent violence.

Obama, speaking from the White House, called for a review of the use of military equipment by local police forces, warning that a continued blurring of the lines between military and local law enforcement would be “contrary to our traditions”.

He struck a more detached tone over the root causes of violence. He acknowledged the feelings of alienation felt by many young African Americans in the US: “In too many communities around this country young men of color are left behind and seen as objects of fear,” he said. But he also accused some protesters of “stirring chaos” and declined to answer whether he would be visiting the city himself.

“While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police, only serves to raise tensions and stir chaos,” said the president in a White House address that interrupted his two-week vacation. “It undermines rather than advanc[es] justice.”

Earlier on Monday, it was revealed that the White House had not been given advance warning of Nixon’s decision to call out the Missouri national guard, something Obama did not deny when asked about the deployment.

“I spoke to Jay Nixon about this, expressed an interest in making sure that if in fact the national guard is used, it is used in a limited and appropriate way,” said the president. “He described the support role that they are going to be providing to local law enforcement. And I’ll be watching over the next several days, to assess whether, in fact, it’s helping rather than hindering progress in Ferguson.”

A heavy-handed police response in the first few days of demonstrations has been widely criticised in Washington, where some lawmakers are proposing legislation to limit a federal scheme that has been used to distribute surplus military equipment to civil police departments.

“I think it’s probably useful for us to review how the funding has gone, how local law enforcement has used grant dollars, to make sure that what they’re purchasing is stuff that they actually need,” said Obama. “Because, you know, there is a big difference between our military and our local law enforcement, and we don’t want those lines blurred. That would be contrary to our traditions.”

But, other than announcing that Holder would be visiting Ferguson on Wednesday, there was little new intervention by the administration in the handling of the crisis.

Unlike his response to the shooting of the Florida teenager Trayvon Martin, Obama has been reluctant to take sides in the dispute over the police shooting of Michael Brown – perhaps wary of further polarising the national debate or appearing to condone the violent reaction.

“I have to be very careful about not prejudging these events before investigations are completed,” said Obama.

Holder acknowledged the frustrations felt by protesters about the lack of information about the investigations. “I realise there is tremendous interest in the facts of the incident that led to Michael Brown’s death, but I ask for the public’s patience as we conduct this investigation,” Holder said in a statement after meeting Obama.

Brown was shot “at least” six times by officer Darren Wilson and was ultimately killed by a bullet that entered the top of his head and travelled “back to front”, attorney Daryl Parks said the autopsy had found, arguing this showed Brown was fatally shot when his head was well bowed.

Dr Michael Baden, who carried out the autopsy, said that “there weren’t signs of a struggle” on Brown’s body. Police have said that Brown assaulted Wilson after the officer stopped him and a friend and told them to walk on the sidewalk rather than in the road.

However Baden, the former chief medical examiner for New York City, was more cautious than the lawyers, stating that the findings in his preliminary report “could be consistent” with suggestions that Brown had been shot while charging at Wilson. “It’s possible,” he said.

“There are many different witness testimonies,” said Baden. “Many seem to line up in one direction, some in another direction. Right now until we get more information we can’t, from a forensic science point of view, distinguish and can’t make an absolute judgment.”

Ferguson was a community ‘asleep at the wheel,’ with residents largely not bothering to vote, often because they did not believe elections would change anything. Michael Brown’s death has changed all that

The killing of Michael Brown by a police officer in Ferguson has sparked violent confrontations and shattered any illusions of racial harmony in Barack Obama’s US. As the standoff continues, Jon Swaine reports on the deep grievances fuelling the protests